There exist at least several ways of obtaining information on the attitude of a satellite: by implementation of specific sensors for the observation of the Sun, of the stars, or of the Earth, or by inertial reference of a gyrometric type. Some examples of this are given in the documents BE-854.174, FR-2.319.150, U.S. Pat. No. 3,535,521, U.S. Pat. No. 3,641,347, U.S. Pat. No. 3,992,106, U.S. Pat. No. 4,628,206 or WO 87/00653.
In the case of a terrestial detection, there already exist a large number of simple specific sensors, but their precision augments with their size, their bulk and their weight. Currently, a detector endowed with sufficient precision to effectively point an optical instrument of an imagery load itself comprises an optical device of a size comparable to this instrument.
The invention aims to do away with the redundance of these optics and to avoid the necessity of providing a specific sensor, which only measures precisely one of the components of attitude variation of a satellite.
To this effect, only one instrument is used for two functions: the instrument serving to obtain the image of the Earth (or another planet or celestial object) by scanning, is modified and completed in a fashion to equally measure the angular velocity of the satellite about an axis parallel to the axis of the line of scanning, with a precision equal to, indeed better than, the angle with which one sees from the satellite the thickness (or step) of the line scanning the ground.
In one embodiment of the invention the modification consists in the addition to the detectors normally provided in the focal plane of the optical scanning instrument of two detectors:
one on the optical axis,
the other, remote from this axis perpendicularly to the scanning lines within limits permitted by the opening of the optical system, by a value d, in a direction opposite to that in which one passes successively from one line to the next.
The first supplementary detector may be done away with if its function may be performed by one of the normal detectors of the focal plane.
The value d is chosen in such a way that, at each instant, the point aimed at on the ground by the second detector is remote from that which is aimed at on the ground by the first one by the exact distance e which is on the ground between two lines, in the direction perpendicular to that of the scanning.
In practice, the known devices for observation by scanning provide North-South scanning by lines orientated East-West, The invention permits in this case the observation of the drift of pitching of such an observation satellite.
In this way, the signal supplied by the second detector in the course of scanning a line is identical to the signal supplied by the first detector in the course of scanning the following line, on condition that the detectors are correctly calibrated and that the sighted object does not move, or moves very little, between one line and the next. In this way, the rotundity of the Earth and the state or thickness of the atmosphere have no influence on the repetition of this signal.
If one measures the instant of passage of the signal from the detectors by a fixed threshold corresponding, for example, to the transition between space and the Earth (in passing through the atmosphere), or more generally between two luminous zones very different in their image, one obtains in the course of each line, for the second detector, a signal which repeats itself identically on the first detector when scanning the following line. This repetition permits permanent tracking of the drift of attitude around the scanning axis, which in turn allows a resetting of, for example, a gyrometer in the satellite.
The invention is applicable in the case where the scanning of a line is instantaneous or sufficiently rapid so that movement of the atmosphere (moving element in the sighted object) is negligible during this time of scanning.
Combinations of two groups of detectors, instead of two detectors, from which one makes an average of the out-going signals, or with the object of redundance, may be used for reducing the calibration errors and/or to obtain more precise information about the angular drift (of the attitude).
In the same way, the fact that the first detector or sensor must be aligned on the optical axis is not at all necessary, the constraint being that the divergence between the two sensors remains equal to the previously defined distance "d" (or a complete multiple of it). In a perfected version of the invention, the sensors are divided into two groups, each group being in the aggregate displaced with respect to the optical axis so as to eliminate the problem posed by the detection of the horizon at the poles (lack of contrast).
In very general terms, the invention proposes an observation method by scanning of a celestial body from a space vehicle comprising a scanning observation system with reference to a scanning axis charaterized by forming successively two signals corresponding to the scanning, in a given scanning direction, of a same band of ground, detecting in each of these signals a characteristic transition representative of a same characteristic point of this band of ground, picking up the relative position of these transitions in these signals, and determining, from any divergence between these relative positions, a possible angular velocity of the space vehicle, taking into account the difference of time separating the instants where each of these transitions appeared in the aforementioned signals.
Thus, this process permits with the same system of observation, the observation by scanning of a celestial body such as the Earth from a space vehicle, and the measurement of the angular velocity of this vehicle around the scanning axis in relation to this celestial body. This results in an appreciable gain in reliability, in simplicity, in bulk, in weight and therefore in available mass.
The invention equally proposes a system of observation of celestial bodies in space, adapted to the implementation of this process, of a type comprising an optical scanning device adapted, with reference to a scanning axis, to scrutinize line by line the celestial body, a detection group in the focal plane of this optical device and along the optical axis of the latter, and an image formation and treatment system connected to this detection group, characterized in that it comprises first and second elements of detection with identical spectral characteristics disposed in the focal plane and displaced parallel to the scanning axis by a distance seen by the optical device from an angle equal to an entire multiple, at least equal to one, of the angle from which this optical device sees the distance between two successive or adjacent scanning lines intercepting the celestial body, as well as a device for angular velocity treatment and measurement connected to each of said detection elements and comprising:
a circuit to read successively, first by one then by the other of these detection elements, the signals representative of a same band of ground scanned on the celestial body, these signals being treated, if necessary, to correspond to one and a same given directional series along this band on the ground:
a circuit to detect in each of these signals a characteristic transition representative of a same characteristic point of the band of ground;
a circuit to read the relative position of each transition in relation to the signal that includes it, and;
a computation circuit adapted to determine, from any difference between these relative positions, an eventual angular velocity of the space vehicle around an axis parallel to the scanning axis, taking into account the time interval separating the instants from which have been read the aforementioned characteristic transitions.
Finally, the invention proposes a space vehicle comprising this system of observation, and angular velocity detectors for only the angular drift of the space vehicle around axes perpendicular to the scanning axis of the observation system.
One will appreciate that it is already known to associate attitude measurements and operations for taking images, for example in the following documents U.S. Pat. No. 4,682,300, U.S. Pat. No. 4,688,091, U.S. Pat. No. 4,688,092 and FR-2.556.159, but this association aims to compensate for the perturbations affecting images due to an eventual drift of attitude.
Further, it is already known through document FR-2.551.201 how to monitor an eventual drift of attitude thanks to a system of observation, but here it is a question of a quite different solution bringing into play a completely different process of forming images, without any scanning in the sense of the invention, with implementation of parallel lines of couple charge detectors (type CCD).